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Drug Resistance in Streptococcus pneumoniae
About Streptococcus pneumoniae is a pathogen that has multiple virulence factors that can cause a plethora of problems within the human host. Virulent strains are encapsulated and have immunoglobulin proteases (IgA-protease) to prevent removal by the immune system as well as pneumolysin that forms pores in host cell membranes. Encapsulation infers that the bacterial cells are able to evade the immune system as well as disseminate through the body. These mechanisms are responsible for causing a high fever, severe chills, and rust colored sputum and may progress to meningitis and/or bacteremia. As it is evident that this microbe can cause substantial harm to the human host, antibiotics have played a huge role in preventing fatal infections by S. pneumoniae. Research History At the beginning of the 1990's there were very high numbers of antibiotic resistant S. pneumoniae strains in the United States. Specifically penicillin, which is a widely used beta-lactam drug. Beta-lactam's are a drug class that contain a ß-lactam ring in their molecular structure and inhibit synthesis of the bacterial cell wall. However, ß-lactamase is an enzyme produced by the bacterial cell to destroy the function of the ß-lactam ring. As this is the typical method of antibiotic evasion from a bacterial cell's standpoint, this concept did not hold true for S. pneumoniae. Research has proven that a unique mechanism is used to cause penicillin resistance in certain strains. The Penicillin Binding Proteins (PBP's) of the bacterial cell, which are the targets of the drug itself, have a drastically lower affinity for binding to penicillin. Hundreds of penicillin resistant isolates from all over the world have proven that this resistance mechanism is unfortunately effective. The Study Using the Streptococcus pneumoniae strain R6 and a transformed strain of R6 via PCR, the researchers isolated PBP gene fragments and tested them with MIC agar plates to make sure the gene was present. The mutations that were found in PBP2x and PBP2b were important because they demonstrated drug resistance. A single point mutation can severely increase the the drug resistance of a strain by lowering the receptor affinity. Examples of these point mutations are PBP2x's 550th amino acid Thr and PBP2b's 446th amino acid Thr. Genetic Analysis The resistant strains of S. pneumoniae contain the "altered" PBP's that have lower affinities for penicillin, as mentioned earlier. Research shows that as many as four out of the five PBP's of S. pneumoniae had large variation in the molecular sizes of the PBP's as well as the quantity of each PBP. The idea of a lower binding affinity suggests that there must be a conformational change of the coded protein. Evidence shows that resistant strains have been able to uptake exogenous DNA via transformation of competent cells as well as Horizontal Gene Transfer (HGT). The altered genetic sequences code for a modified protein, which means that the active site as well as overall structure of the protein must be inherently different. The best explanation of where the exogenous DNA originates from, other than another strain, is Streptococcus viridans. Although this is only a theory, S. viridans is part of the mouth's normal flora which makes it a reasonable prediction. High molecular weight PBP's are typically the target of antibiotics, especially PBP1 and PBP2 in S. pneumoniae. As genetic mutations occur the protein structure is changed, resulting in a lower affinity for penicillin. As seen in the S. pneumoniae strain R6, point mutations that occurred within the penicillin binding domain (specifically PBP2x), the higher the Minimum Inhibitory Concentration (MIC). It was found that based on a given gene, a single mutation has the ability to double the MIC. This can be seen in Figure 1. Often times the increase in MIC is due to altered amino acid residues (within the penicillin binding domain) that are very close to penicillin binding motifs. References Antibiotic Resistance in Streptococcus pneumoniae (Oxford Journals) Penicillin-Binding Protein–Mediated Resistance in Pneumococci and Staphylococci (Oxford Journals) Penicillin-Binding Proteins 2b and 2x pf Streptococcus pneumoniae are Primary Resistance Determinants for Different Classes of Beta-lactam Antibiotics (NCBI) Pneumococcal Infections (Medscape)